1. Field of the Invention
This invention relates to a transparent panel of particular value in buildings, vehicles or other structures where it is desired to admit light while excluding much of the solar energy, thus decreasing the amount of air conditioning or cooling required and conserving energy. This panel comprises a transparent substrate, a thin metal film containing predominantly iron, nickel and chromium, and a metal oxide film containing predominantly the oxides of the same three metals. Preferably, the metal film is deposited by sputtering a suitable alloy in an inert gas, and the metal oxide film is formed by reactively sputtering the same or a similar alloy in the presence of oxygen. This multilayer coating has been found to be highly reflective to infrared radiation while transmitting a desired proportion of visible radiation. Thus, the coating serves to reject a large fraction of the incident solar energy. Further, when the metal oxide film is an overlayer, it provides a durable protective coating for the metal film.
2. Description of the Prior Art
For many years the glass industry has employed several different methods of altering the optical properties of panels used as windows in the automotive and building industries. Some of these methods consist of coating glass or plastic substrates by various methods such as chemical vapor deposition and physical vapor deposition, including sputtering. The optical properties of panels are also modified by altering the composition of the bulk substrate material. Products manufactured by these methods have been only partially successful in reflecting solar radiation to the degree required for significant energy conservation.
Satisfactory coatings for architectural uses are obtained only when the coating process is capable of producing a film of uniform composition and thickness. Further, the process must not warp or otherwise degrade the substrate by overheating. Even when a uniform film is produced it must usually be protected against atmospheric corrosion and abrasion resulting from wind born dirt and periodic cleaning.
Where fragile films are used in architectural panels, the panels are often mounted such that the film is on an interior surface which is not exposed to the weather. Some films must be further protected by mounting them such that the film is on an interior surface of a two panel composite. When a panel is mounted such that the reflecting film is on an inner surface, the solar radiation must pass through the substrate twice, first as it enters and then as it leaves, thus warming the substrate even more than with a single pass.
A fragile metal film is also protected by overlaying the metal film with a film of various metal oxides. U.S. Pat. No. 3,826,728 discloses an architectural panel in which a sputtered film of nickel-base alloy, such as INCONEL 610, is overlayed with a film of silicon oxide or aluminum oxide. U.S. Pat. No. 3,846,152 discloses an architectural panel in which a film of nickel is overlayed with a film of copper oxide.
When both the metal film and the protective material are applied by sputtering, the use of two different sputtering targets is frequently required. Further, two separate power sources are generally required becasue a direct (d.c.) potential is preferred for sputtering a metal target, but a radio frequency alternating (a.c.) potential is preferred for sputtering a non-conductive target, such as aluminum oxide.
In coating a substrate by sputtering, the substrate is placed near a target where it can receive the sputtered material. The system is contained within a chamber which is evacuated, and a quantity of an inert gas, such as argon, is admitted. In d.c. sputtering, an electric potential, generally of several hundred volts, is applied between an anode and the cathode target to produce a glow discharge plasma. Inert gas ions formed in the plasma are attracted to the cathode target where they impact and erode the target material. The sputtered atoms deposit on the substrate and form a coating on the surface.
It has been observed that the chemical composition of the deposit obtained by d.c. sputtering is substantially the same as that of the bombarded target, and that the film adheres to the substrate more strongly than films obtained by such processes as vacuum evaporation.
In reactive sputtering, a gas, such as oxygen, which will chemically react with the target material, is admitted to the chamber during the sputtering process. The substrate becomes coated with the reaction product of the target material and the reactive gas.
Sputtering is ordinarily a relatively slow process, but the rate can be significantly increased by utilizing a magnetic field to confine the glow discharge plasma. One such source is described in an article by John S. Chapin entitled "The Planar Magnetron" which was published in Research/Development January 1974, pp. 37-40.